The present invention relates to a pre-tensioner which rapidly rotates a take-up shaft of a seat belt retractor in the belt take-up direction, the pre-tensioner being incorporated in the seat belt device for restraining an occupant to a seat of a vehicle. More particularly, the present invention relates to a pre-tensioner which can securely impart pre-tension on the belt. The present invention also relates to a pre-tensioner which can effectively use pressure developed by a gas generator or to a pre-tensioner which can be constructed in a compact form as a whole and have lighter weight.
Among seat belt devices installed in vehicles such as automobiles, a seat belt device of a type having a pre-tensioner has been multiplied these days. Such a pre-tensioner rapidly rotates a reel (take-up shaft), on which a seat belt is wound, in the belt take-up direction when a vehicle comes into collision in order to cancel the loosening on the wound belt and impart tension on the belt. In a typical pre-tensioner, powder is ignited according to a signal outputted in the event of the vehicle collision and a reeling mechanism is driven by gas pressure developed by the ignition.
One of such conventional pre-tensioners is disclosed in Japanese Patent Unexamined Publication No. 05-162614.
The pre-tensioner is provided with a pinion fixed to a belt take-up shaft. The pinion has external teeth formed on the outer periphery thereof.
A ring-like pulley separately from the pinion is arranged outside the external teeth of the pinion. The pulley has internal teeth formed on the inner periphery. The internal teeth of the pulley are capable of engaging the external teeth of the pinion. The pulley is accommodated in a housing and is held by pins within the housing. In a state where the pulley is held by the pins, a predetermined clearance between the internal teeth of the pulley and the external teeth of the pinion is ensured. The terminal end of a rope is attached to the outer periphery of the pulley. The rope near the terminal end is wound on the outer periphery of the pulley for several full circles. The start end of the rope is secured to a piston of a power source (gas generator). The piston slides within a cylinder extending straight outside of the housing when the gas generator ignites the powder.
The operation of the conventional pre-tensioner is as follows.
Before the operation of the pre-tensioner, the pulley is held by the fixed pins in a position within the housing where the internal teeth of the pulley and the external teeth of the pinion are not engaged. Therefore, the take-up shaft can freely rotate. As the gas generator is actuated in this state, however, the piston is pushed by gas generated from the gas generator and thus slides within the cylinder, so the rope secured to the piston is pulled. As the rope is pulled, force is exerted on the pulley so as to shear the fixed pins, thereby canceling the holding of the pulley. As a result of this, the pulley on which the rope is wound starts to rotate and move so that the internal teeth of the pulley engages the external teeth of the pinion. Because of this engagement, the rotation of the pulley is transmitted to the pinion and the take-up shaft rotates at the same time as the rotation of the pinion, thereby imparting pre-tension on the belt.
However, the aforementioned pre-tensioner has the following problems:
(1) Since the rope is employed as a member for pulling the pulley, the casing and cylinder must have openings for introducing the rope. There is a problem that the force for biasing the piston becomes weaker because a part of gas from the gas generator leaks through the openings.
(2) Since the cylinder for pulling the rope extends straight outside the casing, the size and weight of the pre-tensioner as a whole are increased.
Another conventional example will be explained.
FIG. 17(a) is an exploded perspective view of the pre-tensioner disclosed in PCT International Publication No. WO95/27638 and FIG. 17(b) is a perspective view showing the details of a drive wheel.
The pre-tensioner comprises a pair of drive wheel halves 103, 104 attached to a take-up shaft 101 for a seat belt as shown in FIG. 17(a). The drive wheel half 104 is secured to the take-up shaft 101. The drive wheel half 103 is assembled to the take-up shaft 101 via the gear 106. Both the drive wheel halves 103, 104 form together a drive wheel 105 as shown in FIG. 17(b) by arranging them to confront to each other. The drive wheel halves 103, 104 are provided with a plurality of concavities 103a, 104a circumferentially formed in the respective confronting surfaces. When the drive wheel halves 103, 104 are arranged to confront to each other, the concavities 103a, 104a form cup-like (hemispheric) concavities 100. Also formed between the drive wheel halves 103 and 104 is a groove 110.
Disposed between the drive wheel halves 103, 104 is a sword box 114. The sword box 114 is attached to a retractor housing (not shown) as well as a cup (casing) 112. The sword box 114 is provided with projecting guide plates 116, 117. The guide plates 116, 117 are positioned in the groove 110 of the drive wheel 105.
Disposed inside the cup 112 is a U-like tube 118. The tube 118 is positioned to surround the sword box 111. The tube 118 has a notch-like opening 118a formed therein to confront a space between the guide plates 116, 117. Parts of the periphery of the drive wheel 105 enter into the opening 118a. 
Disposed on one end (proximal end) of the tube 118, the right upper end in the drawing, is a gas generator 115. The other end (distal end) of the tube 118 is fitted in a throat portion 119 formed in the cup 112. Accommodated in the tube 118 are mass balls 120 for driving the drive wheel 101. The mass balls 120 are a plurality and series of spheres. The mass balls 120 have a piston 121 on the proximal side (the side near the gas generator 115).
The works and problems of the aforementioned pre-tensioner will be described with reference to FIGS. 16(a) through 16(c).
FIGS. 16(a) through 16(c) are front views schematically showing portions surrounding the drive wheel 105 of the conventional pre-tensioner of FIG. 17(a). FIG. 16(a) is a view showing the state before the actuation of the pre-tensioner, FIG. 16(b) is a view showing the state of the normal actuation of the pre-tensioner, and FIG. 16(c) is a view showing the state of the abnormal actuation of the pre-tensioner.
In the state shown in FIG. 16(a) (before the actuation), the head mass ball 120 is not in contact with the drive wheel 105 so that the drive wheel 105 and the take-up shaft 101 can freely rotate. Therefore, no clutch mechanism is required between the drive wheel 105 and the take-up shaft 101.
As the gas generator 115 (FIG. 17(a)) is actuated in the event of an emergency, generated gas presses the mass balls 120 in the tube 118 toward the bottom in the drawing. Thus, the head mass ball 120 advances to the opening 118a and comes in contact with the side of one of the teeth 105a of the drive wheel 105 which is entered into the opening 118a. At this point, the mass ball 120 exerts force in a direction of arrow C, rotating the drive wheel 105.
Since each concavity 105b between the adjacent teeth 105a is formed hemispheric corresponding to the dimension of the mass balls 120, the concavities 105b and the mass balls 120 are engaged in regular order so that the drive wheel 105 rotates. At the same time as the rotation of the drive wheel 105, the take-up shaft 101 rotates to wind up the belt in the belt take-up direction.
In this pre-tensioner, however, the rotation of the drive wheel 105 and the take-up shaft 101 is sometimes not secured. That is, as shown in FIG. 16(c), there is a possibility that the mass ball 120 pressed from the tube 118 collides with one of the summits X of the teeth 105a of the drive wheel. At this point, the force from the mass ball 120 to the drive wheel 105 acts in a direction of arrow D, that is, in the direction toward the axis of the drive wheel 105. In this case, the force of a torque of the drive wheel 105 created by mass ball 120 is reduced, so the efficiency of retracting the belt is considerably reduced. This means that the operation efficiency of the pre-tensioner disclosed in WO95/27638 is unstable.
The present invention was made in order to solve the problem and the object of the invention is to provide a pre-tensioner which can securely impart pre-tension on the belt, a pre-tensioner which can effectively use ignition pressure of a gas generator or a pre-tensioner which can be constructed in a compact form as a whole and have lighter weight.
In order to solve the above-described problem, a pre-tensioner of the present invention is a pre-tensioner which rotates a take-up shaft of a seat belt retractor in the belt take-up direction to impart pre-tension on a belt in case of emergency, the pre-tensioner comprising: a gas generator; a series and plurality of driving members which are accelerated by gas from the gas generator; a passage for guiding the driving members; a first rotational member having a plurality of driving points (levers or the like) which are pressed by some of the driving members to receive a driving force; a second rotational member fixed to the take-up shaft; and a clutch mechanism disposed between the first rotational member and the second rotational member, wherein the clutch mechanism is not coupled before the pre-tensioner is actuated, while as the first rotational member rotates when the pre-tensioner is operative (such as in a vehicle collision), the clutch mechanism is coupled, and wherein the head one of the driving members is substantially in contact with one of the driving points of the first rotational member.
When the pre-tensioner is inoperative (the normal state), the clutch mechanism is not coupled so that the first rotational member and the second rotational member are not coupled. Therefore, the second rotational member and the take-up shaft operate independently of the pre-tensioner.
When the pre-tensioner is operative, the gas generator ignites to accelerate the driving members so that the head one of the driving members presses the lever of the first rotational member. Since the head driving member is substantially in contact with the driving point of the first rotational member, the driving point is securely pressed. As the lever is pressed, the clutch mechanism is coupled so that the second rotational member rotates. According to the rotation of the second rotational member, the take-up shaft rotates to wind up the belt.
Since the head driving member is substantially in contact with the driving point of the first rotational member, the driving point of the first rotational member can be pressed just after the ignition of the gas generator. This securely prevents incorrect engagement between the driving member and the first rotational member, thereby securing the operation of the pre-tensioner. The expression xe2x80x9csubstantially in contact withxe2x80x9d means that there may be a space not harming the operation between the head driving member and the driving point of the first rotational member.
In the present invention, the second rotational member may be a pinion having external teeth, the first rotational member may be a ring gear having internal teeth engageable with the external teeth of the pinion and having the levers on the outer periphery thereof, and the clutch mechanism may engage the internal teeth of the ring gear with the external teeth of the pinion so that the accelerated driving members press the lever of the ring gear to move the ring gear.
The transmission of the rotation from ring gear to the pinion speeds up the rotation of the pinion. No expensive part is employed, thereby facilitating the structure of the device as a whole and reducing the production cost.
Also in the present invention, the driving members are preferably spherical bodies (balls), and the passage is preferably in a pipe-like shape and is bent.
Accordingly, the passage of the driving members can be bent. The passage may be bent two- or three-dimensionally, reducing the size of the pre-tensioner. The degree of freedom for design is improved as compared to the conventional one having a straight pipe. The driving members may be connected like a string of beads or may be separated.
In the present invention, further, the bottom one (nearest to the gas generator) of the driving members preferably has a sealing function for preventing gas leakage toward the head one, and wherein the other driving members are preferably relatively loosely fitted in the passage.
Since the gas is sealed, the efficiency of the gas is improved. Because the other driving members are loosely fitted, the driving members can smoothly move.